DE69009267T2 - Combined heat exchange system for ammonia synthesis reaction streams. - Google Patents

Description

The present invention relates to heat exchangers and, in particular, to heat exchanger systems in which a stream of process fluid passes through 2 consecutive heat exchange stages where heat is exchanged between the fluid stream and 2 exchange fluids and in which the operating conditions or physical properties, such as temperature, corrosiveness, etc. of the fluid stream are aggressive to conventional materials of construction.

The sequential exchange of heat between a fluid stream and 2 or more exchange fluids is practiced in many processes. The most common arrangement involves a separate shell and tube heat exchanger for each exchange fluid, with the fluid stream flowing through successive heat exchangers via connecting tubes. In a typical shell and tube heat exchanger, the fluid stream can flow through either the shell side or the tube side of the exchanger, with the best choice depending on the specific circumstances.

An example of a sequential heat exchange is an ammonia synthesis system as described in US-A-4744966 wherein effluent gas (fluid stream) from a reactor is cooled in a first heat exchanger with a gas fed into the reactor and then in a second exchanger by exchanging with steam in which the fluid stream flows from the first exchanger via a connecting line to the second exchanger.

When the conditions or properties, such as temperature, corrosivity, etc. of the process fluid are aggressive with respect to common construction materials, various Arrangements are practiced to protect the casing of an exchanger against such an aggressive fluid.

According to the specific example of US-A-4744966, the effluent gas from the first reactor leaves the reactor at a temperature of between 480 ºC and 540 ºC and is passed into a first heat exchanger where it is cooled to a temperature of between 390 ºC and 440 ºC by exchanging with the gas fed into the first reactor. The effluent gas is then passed through a connecting line into a second heat exchanger where it is further cooled by exchanging with steam. In this application, the gas effluent from the reactor is normally passed into the tubes of the first exchanger by means of a connection such as that described in US-A-4554135 in order to avoid exposing the envelope to the aggressive incoming gas. In the second exchanger, for economic reasons, the gas is led through the side of the exchanger casing and the steam through the tube side.

It has been recognized by the inventors that in the lower part of the effluent gas temperature range, namely below about 400 ºC, a thin-walled connecting pipe made of chromium-molybdenum alloy steel can be used. In the upper part of this temperature range, however, in order to avoid nitriding of the metal, a more expensive design is required, using a much thicker pipe wall or a coating within the pipe made of a nitriding-resistant material such as Inconel. Regardless of the design, at such high temperatures, difficulties arise with thermal expansion, differential expansion, maximum load and protection of welded joints, including mold closure in welded designs.

The present invention relates to heat exchangers and is particularly applicable to heat exchanger systems in which a fluid stream is passed through 2 successive heat exchange stages in which heat is exchanged between the fluid stream and 2 exchange fluids and in which the operating conditions or physical properties, such as temperature, corrosiveness, etc. of the first fluid are aggressive towards common construction materials. According to the invention it has been recognized that the aggressiveness of the fluid varies as the fluid flows through the exchanger system. According to the invention the advantage of the varying aggressiveness is used and the problems associated with the aggressiveness are avoided or reduced and the equipment costs are reduced.

According to one aspect of the present invention there is provided a heat exchange device for effluents of an ammonia reactor, comprising an inflow-outflow exchanger comprising a first tubular exchange zone containing a bundle of heat exchange tubes held within the inflow-outflow heat exchange zone; and a high temperature heat removal device comprising a second tubular exchange zone containing a bundle of heat exchange tubes; characterized in that the heat exchange device further comprises a shell, the heat exchange tubes of the first tubular exchange zone and the heat exchange tubes of the second tubular exchange zone being located within the shell; a tube sheet separating the inflow-outflow heat exchanger from the high temperature heat removal device, and the heat exchange tubes of the first tubular exchange zone terminating in said tube sheet so that the high temperature heat removal device is connected directly in series with the inflow-outflow exchanger; and a protective shield between the cladding and the heat exchange tubes of the second tubular exchange zone to protect the cladding in the high temperature heat removal device near the tube sheet.

According to another aspect of the invention, there is provided a method for a heat exchanger system for cooling or heating a fluid stream of a process fluid, comprising the steps of: passing the fluid stream through 2 exchange stages connected in series, comprising exchanging heat between the fluid stream and a first exchange fluid by passing the fluid stream through a tube side of a single-pass bundle and exchanging heat between the fluid stream and the second exchange fluid by passing the fluid stream through a shell side of a multi-pass bundle; characterized in that the first and second tube bundles are in a single shell; and in that the method further comprises forming a seal between the shell and a tube sheet of the single-pass bundle to separate fluids in different shell sides of the 2 heat exchange stages; and arranging a flow sheet such that the operating conditions and/or physical properties of the process fluid in the jacket side of the multi-pass bundle are more aggressive at an end closer to the tube sheet of the single-pass bundle than at an end further from the single-pass bundle; and protecting the jacket from the process fluid at the more aggressive condition of the process fluid by interposing a protective shield between the jacket and at least a portion of the multi-pass bundle at the end closer to the tube sheet of the single-pass bundle.

Embodiments of the invention will now be described by way of example, with reference to the accompanying drawings, in which:

Figure 1 illustrates a schematic representation of a combined heat exchanger system according to the invention as used for cooling the effluent of an ammonia reactor;

Figure 2 is a detailed cross-section of a combined heat exchanger including a protective coating;

Figure 3 is a cross-section of an alternative combined exchanger with a shield plate;

Figure 4 is a cross-section of another alternative combined exchanger with a shield plate;

Figure 5 illustrates the cross-section of a portion of a combined heat exchanger incorporating a sealing bushing;

Figure 5a is a detailed view of the sealing bushing of

Figure 5; and Figure 5b illustrates in cross-section a portion of a combined heat exchanger containing a heat bushing in combination with a sealing bushing.

The preferred embodiments will now be described with reference to the drawings. For ease of description, a number referring to a component in one figure refers to the same component in another figure.

Figure 1 is a schematic representation of a combined exchanger 10 according to the invention, which is used, for example, to cool down effluents from an ammonia reactor 5. The ammonia reactor 5 rests vertically on the reactor support 7 and is connected to the combined exchanger 10. The combined exchanger 10 is horizontally oriented and supported by a strut 9, which allows axial movement to accommodate thermal expansion.

Figure 2 illustrates details of a combined exchanger 10. The combined exchanger 10 includes a first upstream-downstream exchange zone 20 and a high temperature heat removal device 50 within the shell 22, the 2 zones being separated by a tube sheet 36. Hot reactor gas from the reactor 5 flows through a line 44 into an exchange head 40 of a first exchange zone 20 through an expansion element, such as an expansion joint 42. The first exchange zone 20 includes a tube bundle 30 extending between the tube sheets 36 and 38. The hot reaction gas flows through the tubes 32 and the tube sheet 36 into the shell side of the second exchange zone 50.

The reactor feed gas flows into the shell side of the first exchange zone 20 through the inlet 12 where it is heated by exchanging with the hot reaction gas within the tubes 32. Feed gas is circulated through baffles 34 on its way through the first exchange zone 20. After being heated, the feed gas flows through a shell tube 14, thereby entering the reactor 5. In this example, the temperature of the reaction gas leaving the first exchange zone 20 is in the range of approximately 425°C to 440°C. The second exchange zone 50 is constructed of a U-tube bundle 52 having a plurality of U-tubes 54. The reaction gas flows along the shell side of the U-tube bundle 52 and exits at gas outlet 58. The reaction gas is cooled by heat exchange with the steam within the U-tubes 54, forming a high temperature superheated steam. Low temperature steam enters the U-tubes 54 via a steam inlet 62 and exits via an outlet 64. The U-tubes 54 are held within a tube sheet 56. Incoming steam entering a head zone 60 above the inlet 62 remains separated from the outlet 64 by the flow divider 66.

Although the illustrated embodiment of the high temperature heat removal device consists of a steam superheater, other high temperature fluid exchangers may be used, including a boiler feed water exchanger, a boiling water exchanger, or the like.

In the example of Figure 2, the temperature of the effluent gas entering the second exchange zone 50 is in a temperature range of 425 to 440 ºC. In order to protect against nitriding in this range, means are provided for protecting the part of the cladding closest to the tube sheet 36. A coating 70 of a nitriding resistant material, such as Inconel, is applied along the inner surface of the cladding 22 from the tube sheet 36 for a sufficient distance along the length of the second exchange zone 50 where the effluent gas has been sufficiently cooled by heat exchange with the steam within the U-tubes 54 to a point where the effluent gas is safely at a temperature below about 425°C, which is a maximum temperature for the nitriding operations for which a thick-walled chromium molybdenum alloy is suitable. In a more conservative embodiment, the coating 70 extends over a substantial portion of the length of the steam superheat zone 50.

The steam superheat zone 50 may also include a cleanout port 57 in the enclosure 22 downstream of the tubesheet 36 to allow access to the tubesheet 36, e.g., for inspection or maintenance. The cleanout port 57 is sealed by the cover 57a. Although not illustrated, both the cleanout port 57 and the cover 57a may be coated with a nitriding resistant material.

Figure 3 illustrates an alternative combined exchanger 10a in which an alternative second exchange zone 50a includes a different enclosure protection of a shielding plate 80. With the exception of the protection in the form of the shielding plate 80 instead of the coating 70 of Figure 2, the design elements of the combined exchanger 10a of Figure 3 are identical to those of Figure 2 and therefore need not be repeated.

The shield plate 80 consists of a cylindrical protective wall attached to the tube sheet and extending axially between the cladding 22 and the U-tube bundle 52. The shield plate 80 isolates hot reaction gas exiting the tubes 32 of the first exchange zone 20 from the hotter part of the outer cladding 22. The shield plate 80 is supported at the downstream end by the supports 82.

Figure 4 illustrates a more conservative alternative in which the shroud 80' extends most of the length of the tube bundle 52 and an outlet nozzle 58 is located near the gas inlet end of the exchange zone 50. The example shown in Figure 4 provides an annular passageway between the shroud 80 and the enclosure 22 through which the cooled effluent gas flows, thereby keeping the enclosure 22 temperatures safely below 425°C. The shroud 80' is supported at the downstream end by the supports 82'.

The steam superheat zone of any of the embodiments may include separation flanges for disassembly. In the example illustrated in Figures 3 or 4, the alternative steam superheat zone 50a includes flanges 51a and 51b as shown by the dashed lines, whereby the steam superheat zone 50a can be removed to allow access to the tubesheet 36.

Although not illustrated together, the coating 70 of Figure 2 may be combined with the shielding plate 80 of Figure 3 (or with the shielding plate 80' of Figure 4) to combine to protect the portions of the enclosure 22 exposed to the high temperature effluent gas.

Also included is a means for sealing the cladding side of the first exchange zone 20 from the cladding side of the second exchange zone 50 to keep the fluids separated. In one example, the tubesheet 36 is welded or otherwise attached to the cladding 22, as in Figures 2, 3 or 4. A means for allowing thermal expansion of the tube bundle 30 may also be included, such as an expansion joint.

The preferred means for sealing the tube sheet 36 to the enclosure 22 is the sealing bushing 90 illustrated in Figures 5 and 5A. Such sealing means also enable thermal expansion of the tube bundle 30.

The sealing bushing 90 seals and isolates the tube sheet 36 from the exchanger casing 22. The tube sheet 36 is provided with a cylindrically extending portion 36a defining an annular space therebetween in which the sealing rings 92 are disposed. The clamp ring 95 is attached to the extended portion of the tube sheet by means of the pin 94 which is secured by means of the toggle nut 96. The sealing pin 98 passes through the sealing ring 95, thereby adjustably compressing the sealing rings 92 to give the desired sealing pressure.

Figure 5B illustrates another embodiment in which a thermal bushing 99 is combined with a sealing bushing 90a at the outer end of the thermal bushing 99 (the structural elements of the sealing bushing are the same as those of the sealing bushing 90 of Figure 5A and are therefore not repeated). This arrangement is particularly advantageous because it is flexible and subject to lower bending stresses, thereby providing a better and more secure seal.

Although the illustrated embodiments in the two-passage bundle use U-bends as a means of returning the fluid from the first passage to the second passage, other means may be used, such as a floating head.

In another embodiment, a bundle of passages with an even number, e.g. a four-passage bundle, may alternatively be used instead of the two-passage bundle.

The combined exchanger arrangement illustrated and described eliminates the need for a connecting line between adjacent exchangers. Such a connecting line is subject to thermal stress and may require a nitriding resistant coating along its length. By combining the exchangers within a single enclosure, the difficulties due to high thermal expansion stresses and the problems of obtaining secure welds with a connecting line that is either coated or made entirely of a nitriding resistant material are reduced. Elimination of the connecting line also reduces the amount of material exposed to the high temperature effluent. In one case, it was found that using the combined exchanger arrangement saved approximately $200,000 compared to an arrangement of 2 separate exchangers.

In the embodiments described above, the fluid stream enters the exchangers at an elevated temperature and is cooled as it passes through the tubes of the single-pass tube zone and then through the cladding side of the two-pass tube zone. In another embodiment, the fluid stream enters the exchanger at a relatively mild temperature and is heated to a more aggressive state as it passes through the cladding side of the two-pass tube zone and then through the tube side of the single-pass tube zone, with means for protecting the cladding and the two-pass tube zone similar to those described in the previous embodiments.

In another embodiment, the fluid stream enters or leaves the exchanger at an aggressive cold temperature, the exchanger being designed with protective means as described above for hot gases.

The heat exchanger system described is therefore particularly suitable for heat exchange with fluids in which the conditions or properties, such as temperature, corrosiveness, etc., of the first fluid are aggressive towards common construction materials, and in which the aggressiveness of the fluid varies as the fluid flows through the exchanger system, and is particularly suitable for cooling the effluent of an ammonia reactor.

Claims (16)

1. Heat exchange device for effluents from an ammonia reactor comprising: an inflow exchanger comprising a first tubular exchange zone (20) containing a bundle (30) of heat exchange tubes (32) held within the inflow heat exchange zone; and a high temperature heat removal device comprising a second tubular exchange zone (50, 50a) containing a bundle (52) of heat exchange tubes (54); characterized in that the heat exchange device further comprises an enclosure (22), the heat exchange tubes (32) of the first tubular exchange zone and the heat exchange tubes (54) of the second tubular exchange zone being located within the enclosure; a tube sheet (36) separating the inflow heat exchanger from the high-temperature heat removal device, and the heat exchange tubes (32) of the first tubular exchange zone terminating in this tube sheet (36) so that the high-temperature heat removal device is directly connected in series with the inflow exchanger; and a protective shield (70, 80, 80') between the casing (22) and the heat exchange tubes (54) of the second tubular exchange zone for protecting the casing in the high temperature heat removal device near the tube sheet (36). 2. Device according to claim 1, characterized in that the protective shield comprises an inner casing (80, 80') located between the casing (22) and the tubes (54) of the second tubular exchange zone (50a). 3. Device according to claim 2, characterized in that the inner protective casing (80) extends along the second tubular exchange zone (50a) over a length sufficient to protect the part of the casing (22) which would otherwise be exposed to unacceptably aggressive conditions. 4. Device according to claim 2, characterized in that the inner protective casing (80') extends along the second tubular exchange zone (50a) over substantially its entire length. 5. Device according to one of the preceding claims, characterized in that the tube base (36) is attached to the casing (22). 6. Device according to claim 1, characterized in that the protective shield comprises a protective coating (70) provided on at least a portion of the inner surface of the enclosure (22) in the second tubular exchange zone (50) which would otherwise be exposed to unacceptably aggressive conditions. 7. Device according to claim 6, characterized in that the protective shield further comprises a heat trap (80, 80') which is mounted between the casing and the tubes (54) of the second tubular exchange zone (50a). 8. Device according to one of the preceding claims, characterized in that the protective shield (70, 80, 80') consists of a nitriding-resistant material. 9. Device according to one of claims 1 to 7, characterized in that the protective shield (70, 80, 80') consists of INCONEL (trademark). 10. Device according to one of the preceding claims, characterized in that the high-temperature heat dissipation device comprises a high-temperature fluid exchanger. 11. Device according to one of claims 1 to 9, characterized in that the high-temperature heat removal device is selected from the group consisting of: steam superheater, steam boiler feed water exchanger and boiling water exchanger. l2. Device according to one of the preceding claims, characterized in that the casing is divided into 2 sections, with connecting means (51a, 51b) between the inflow exchanger and the high-temperature heat removal device. 13. Device according to one of the preceding claims, characterized in that it further comprises a sealing bushing (90) for sealing the tube sheet (36) with the envelope (22). 14. Device according to claim 12, characterized in that the connecting means comprise contact flanges (51a, 51b). 15. Device according to one of the preceding claims, characterized in that the heat exchange tubes (54) of the second tubular exchange zone (50, 50a) comprise a multi-pass bundle with a uniform number of passages. 16. A heat exchange system method for cooling or heating a fluid stream of a process fluid, comprising the steps of: passing the fluid stream through 2 heat exchange stages connected in series, comprising exchanging heat between the fluid stream and a first exchange fluid by passing the fluid stream through a tube side of a single-pass bundle (30) and exchanging heat between the fluid stream and a second exchange fluid by passing the fluid stream through a shell side of a multi-pass bundle (52); characterized in that the first and second tube bundles are in a single shell (22); and in that the method further comprises forming a seal between the shell and a tube sheet (36) of the single-pass bundle (30) to separate fluids into different shell sides of the 2 heat exchange stages; and aligning a flow pattern so that the operating conditions and/or physical properties of the process fluid in the shell side of the multi-pass bundle are more aggressive at an end closer to the tube sheet (36) of the single-pass bundle than at an end further from the single-pass bundle (30); and protecting the shell from the process fluid in the more aggressive state of the process fluid by interposing a protective shield (70, 80, 80') between the shell and at least a portion of the multi-pass bundle (52) at the end closer to the tube sheet (36) of the single-pass bundle (30).